The objective of this proposal is to examine the relationships between opioid peptide neurons and other dorsal horn elements associated with nocieptive control, at the ultrastructural level. To identify the afferents to spinal opioid neurons, we will use PAP immunocytochemistry to localize opioid peptides, and combine this localization with autoradiographic detection of either descending monoamine or primary afferent axons. Radiolabeled primary axon terminals result from transport of H3-leucine from dorsal root ganglia; monoamine axons and primary afferent terminals, containing either substance P or VIP, are rediolabeled by radioimmunocytochemistry. To identify which of the anatomically diverse neurons of the superficial dorsal horn receive opioid peptide input, we will combine a Golgi-EM technique with opioid immunocytochemistry. These double labeling studies will enable us to evaluate the relationships between pain transmission and pain control systems in the dorsal horn, with particular emphasis on the anatomical substrate underlying opioid-generated nociceptive control. In addition to the studies of the peptidergic organization of the spinal dorsal horn, we propose to evaluate the opioid peptide inputs to physiologically characterized neurons of the midbrain periaqueductal grey of the rat. We will intracellulary mark neurons and then use immunocytochemical techniques to identify opioid peptide inputs. This will provide important information as the mechanisms through which opiates generate analgesia by interactions in the PAG and will provide information about the relevant endogenous opioid peptides which contribute to that control.